Variable density light-filtering means utilizing stannous chloride and thiazine dye



Oct. 29, 1968 Filed April 22. 1963 PERCENT TRANSMITTANCE PERCN TTRANSMITTANCE R J. HOVEY 3,408,134- VARIABLE DENSITY LIGHT-FILTERINGMEANS UTILIZING STANNOUS CHLORIDE AND THIAZINE DYE 5 Sheets-Sheet 1 soso Mp/1cm ATTORNEY Oct. 29, 1968 R. J. HOVEY 3,403,134

VARIABLE DENSITY LIGHT'FILTERING MEANS UTILIZING STANNOUS CHLORIDE ANDTHIAZINE DYE Filed April 22, 1963 3 Sheets-Sheet. 2

PERCENT TRANSMI TTANCE 400 450 500 550 600 650- 700 380 a (MAD PERCENTTPANSMITTANCE A ('m A) INVENTOR.

RICHARD J. HOVEY ATTORNEY Oct- 29, 1968 R J. HOVEY 3,408,134

VARIABLE DENSITY LIGHT-FILTERING MEANS UTILIZING STANNOUS CHLO THIAZINEDYE 3 Sheets-Sheet 3 RIDE AND Filed April 22, 1963 INVENTOR.

RICHARD JHOVEY ATTORNEY United States atem VARIABLE DENSITYLIGHT-FILTERING MEANS UTILIZlNG-STANNOUS CHLORIDE AND THIA- ZINE DYE.

Richard J. Hovey, Worcester, Mass., assignor to American OpticalCompany, Southbridge, Mass., a voluntary association of MassachusettsFiled Apr. 22, 1963, Ser. No. 274,415 8 Claims. (Cl. 350160) The fieldof this invention is that of light filters of variable optical densityand the invention relates more particularly to novel and improved lightfiltering means having the property of automatically varying in opticaldensity in response to variation in the light incident upon thefiltering means.

Light filters of variable optical density can be very usefulparticularly where flash conditions or variations in sunlight or thelike make protection against frequent or sudden changes in lightintensities desirable. For example, variable density light filters canbe employed in protective spectacle lenses and the like for permittingvariation of optical density as the wearer moves between sunlight andshade or between daylight and artificial light or as the wearer isexposed to flashes of light and the like in various industrial andmilitary environments. However, prior to the present invention, manysuch potential applications of variable density light filters have notbeen practical for the reason that no efficient and economical meanshave been available for initiating and regulating the variation offilter density.

It is an object of this invention to provide novel and improved variabledensity light filtering means; to provide such filtering means whichhave the property of automatically varying in optical'density inresponse to variation in the light incident on the filtering means; andto provide such filtering means in which the variation in opticaldensity is proportional to the variation in said incident light. It is afurther object of this invention to provide variable density lightfiltering means which can be substantially completely transparent underselected lighting conditions and which can acquire substantiallight-absorbing properties in response to the increasing in cidence oflight thereon; to provide such filtering means which can be veryresponsive to increasing incidence of light thereon for rapidlyacquiring or increasing its lightabsorbing properties; and to providesuch filtering means which can be very responsive to decreasingincidence of light thereon for rapidly decreasing or eliminating itslight-absorbing properties. It is an additional object of this inventionto provide light filtering means of variable optical density which canbe varied in optical density a multiplicity of times; to provide suchfiltering means which can display uniform light-absorbing propertiesover its effective area; to provide such light filtering means which areof light, compact, rugged and inexpensive construction; and to providesuch light filtering mean which are adapted for use as protectivespectacle lenses and the like.

Other objects, advantages and details of the variable density lightfiltering means provided by this invention appear in the followingdetailed description of preferred embodiments of the invention, thedetailed description referring to the drawings in which:

FIGS. 1-4 inclusive are graphs illustrating variations in thelight-absorbing properties of light filtering means provided by thisinvention; t

FIG. 5 is'a plan view of-a variable density light filter provided bythis invention;

FIG. 6 is an enlarged section view along line 66 of FIG. 5; and

FIG. 7 is a partial section FIG. 6.

view along line 77 of 3,4fi3,134 Patented Oct. 29, 1968 In accordancewith this invention, a leucobase of a thiazine dye such as Azure C canbe formed in a suitable solvent with a quantity of the reversiblereducing agent stannous chloride. This solution can be photochromic inthat the optical density or light-absorbing properties of the solutioncan change in response to variations in irradiation of the solution withlight of selected wavelengths. For example, the dye Azure C can beplaced in aqueous solution in a concentration of 9.26 10- M togetherwith stannous chloride in concentration of 5.64 10- M, the solutionbeing acidified by the addition of hydrogen chloride in concentration of0.33 M. This solution can be highly transmissive in a room having moreor less average artificial illumination. However, when irradiated withultraviolet and short wavelength visible light of relatively greaterintensities, the solution rapidly acquires a pronounced blue color anddisplays substantially increased light-absorbing properties. Forexample, a sample of this solution having a thickness of approximatelyone centimeter can have absorptive and transmissive propertiessubstantially as indicated by curve a in FIG. 1 in a room having more orless average artificial illumination. (Actually, cunve a represents theabsorptive and transmissive properties of the sample solution underslightly higher than average illumination conditions within apparatusfor testing the absorptive and transmissive properties of the solution.)When this sample of the solution is irradiated at a spacing of onecentimeter for a period of one minute by two four-watt ultraviolet lampssuch as those sold by General Electric Company under the designation BLBlamps, the solution acquires absorptive and transmissive propertiessubstantially as indicated by curve b in FIG. 1. When returned to theaverage room illumination conditions above described for a period ofabout vfour and one-half minutes, the sample of solution can return toits highly transmissive state and can display absorptive andtransmissive properties substantially as indicated by curve 0 in FIG. 1.It should be noted that these reactions of the solution sample are thosewhich occur at room temperature but that change in temperature of thesolution merely alters the rate of the described reactions. An increasein temperature, for example, will result in a decrease in the rate ofcolor build-up in the solution during irradiation and will result in anincrease in the rate of return of the solution to its highlytransmissive state under average illumination conditions.

Although it is not intended to limit the scope of this invention byreference to any theory as to the chemical reactions which may takeplace in the solution above described, it is believed that stannous ionsare oxidized in solution to form stannic ions and that, although stannicion does not react with the reduced or leuco form of the thiazine dyeunder the average illumination conditions described, the stannic ion canreact to oxidize the dye to a colored state when the dye has beenexposed to or excited by ultraviolet and near ultraviolet light.

The concentration of leucobase thiazine dye which is employed in thesolution can be substantially reduced while still achieving asignificant degree of the described photochromic effect. For example,the thiazine dye Azure C can be placed in an aqueous solution inconcentration of 2.81 X 10 M with stannous chloride in concentration of3.10 lO M, the solution being acidified by the addition of hydrogenchloride in concentration of 0.30 M. Under the average artificiallighting conditions above described, a one centimeter thick sample ofthe solution can have the absorptive and transmissive propertiessubstantially as indicated by curve d in FIG. 2. After irradiation ofthe solution sample by ultraviolet lamp in the manner also abovedescribed, the solution sample can display absorptive and transmissiveproperties as indicated by curve j 3,408,134 at E1 e in FIG. 2. Afterreturn of the sample solution to the tially increases the rate atwhich'the' solutions"'return'to noted average illumination conditionsfor a period of their more highly transmissive state under averageilluabout four and one-half minutes, the sample can display minationconditions. Actually, it has been found that the the propertiesindicated by curve f in FIG. 2. aqueous solutions cannot achievesuit-able variations in Conversely and preferably, the concentration ofleuco- 5 Optical density in response to'variations of incident lightbase thiazine dye which is employed can be substantially unless the pHof the solutions is at least less. th'anabout increased to saturation ofthe solution for increasing the 1.3. For example, an aqueous solution ofa.thiazine dye photochromic effects and the rate of the photochromicAzure Bsin concentration of 2.87Xl- Mandtstannoll effects which can beobtained. For example, the thiazine chloride in concentration'of6.8ll()-; .M.wasqfound to dye Azure C can be placed in aqueous solutionfor saturatdisplay suitable photochromic effects wherefthe solution ingthe solution and stannous chloride can be added for had been acidifiedby the addition of hydrogen chloride achieving a concentration thereofof 918x10" M, the in a concentration of only 0.05M. 'The requirement ofsolution being acidified by the addition of hydrogen chlorelatively highconcentrations of hydrogen ion in the ride in concentration of 0.40 M.Under the noted average aqueous solutions provided by this invention isbelieved artificial lighting conditions, a one centimeter thick samto"be explained in that such hydrogen ion concentrations ple of thesolution can be substantially colorless as incan reduce any tendency forhydrolysis of stannic ions dicated by the curve g in FIG. 3. Whenirradiated by ultrain the solutions, thereby permitting more rapidreaction violet lamps in the manner above described, the sample betweenthe stannic ions and the selected dyes wh n the of solution displays theabsorptive and transmissive propsolutions are irradiated withultraviolet light. I erties as indicated by curve h in FIG. 3. Further,after It should be noted that variable density light filtering return tothe noted average illumination conditions for solutions can be preparedwith leucobase thiazine dyes by a period of only three and one-halfminutes, the sample employing other solvents than water. For example,where of solution can substantially completely return to its colorthethiazine dye Azure A has been placed in alcohol soluless states asindicated in curve in FIG. 3. In this solution in a concentration of5.00 10- M with stannous tion, it can be seen that the absorptiveproperties can 'be chloride in concentration of 5.00 l(l M, the solutionsubstantial under the full irradiation described and in fact hasachieved adequate photochromic effects. For example, the sample ofsolution was seen to acquire a very proa one centimeter thick sample ofthis solution can display nounced blue color almost immediately afterirradiation the absorptive and transmissive properties indicated by ofthe sample by the ultraviolet lamps had begun. Furthe curve k in FIG. 4under average artificial illumination ther, the rate of the return ofthe solution to a su-bstanconditions such as have been previouslydescribed. Howtially colorless state can also be seen to be quite rapid.ever, when exposed to ultraviolet light in the manner Other leucobasethiazine dyes can be employed for above described, the solution hasrapidly acquired the forming the desired variable density lightfiltering means absorptive and transmissive properties indicated by theas seen by reference to the following Examples A-F incurve 1 in FIG. 4.Further, when returned to the noted elusive. average illuminationconditions for a period of about eight Example A B 0 D E F ToluidineblueO 8.47Xl0- M New methylene blue. 6 92 10 Methylene blue 8.56Xl0- MThionine- Saturation AzureA 4.29Xl0- Stannouschlorlde. 3.44X103M2.81X10-M 2.95X10-3M 2.86X1O-3M 2. 07 10-3M 2.86X10-3M Hydrogen chloride0.33M 0.25M 0.29M 0.20M 0.20M 0.29M

These solutions each display photochromic effects simminutes, thesolution has displayed properties as indicated ilar to those describedabove with reference to solutions by the curve ml in FIG. 4. It shouldbe noted that such embodying the dye Azure C. Thus each of thesesolutions alcohol solutions can be substantially neutral or acidic canserve as variable density light filtering means accordwithout retardingthe described photochromic effects. ing to this invention. 5 It must beunderstood that the chemical reaction which It has been noted above thatincreasing the concentratakes place in the above noted photochromicsolutions is tion of the leuco thiazine dye in a solution tends toinbelieved to comprise a reversible photo-oxidation process. crease therate at which the solution can respond to irra- Accordingly, it isextremely important that allatmosdiation with ultraviolet light foracquiring increased lightpheric oxygen and the like be excluded from thechemical absorbing properties. This can be explained in that more systemto assure that described chemical reactions can leuco dye in such asolution can be exposed to the exciting occur repeatedly in response tovariations in incident light light. It is also believed that thedescribed photochromic over a long period of time. As shown in FIG. 5, acell 10 solutions should properly include at least stoichiometl'ie canbe formed of an annular member 12 and two flat disc quantities ofstannouschloride for assuring that all of the members 14 each f hi h bdi a i bl i h y in Solution can P p y Teactfid With stannOuStransmitting material. These light-transmitting members chloride. It hasalso been empirically noted that increascan be adhered together with asuitable transparent ing the concentration of stannous chloride in asolution ment to form the cell for enclosing .orencapsulafing a cansubstantially increase the rate at which a solution can quantity of aphotochromic solution. 16 therein; Prefep return to its moretransmissive state when the noted average illumination conditions havebeen restored. Prefer- 5 ably, in order to obtain a reasonably rapidrate of such return of the solution to its more transmissive state, theconcentration of stannous chloride in solution should be at least fivetimes greater than the concentration of dye ably, the annular member 12has an aperture18 therein through which the solution 16 can beintroduced to the cell, this aperture being sealed with the plug '20"and a suitable cement after filling of the cell as will be 'under stood.Various light-transmitting materials and, cements can be employed forforming the cell but preferably, the

in the solution.

In the variable density light filtering solutions thus far u s h 4 m andth 1 1 5 933 t lli described, suflicient hydrogen chloride has beenincluded of y t y methacrylate d se er c i e to assure that the solutioncan have a pH of less than 1.0. d e it an ethylene chlorlne c l I uld- Ithi rd, it ha b e fou d th t increasing acidity understood that althougha thin flat circular cell has been in aqueous solutions provided by thisinvention subst-anillustrated, the cell could be rectangular in outlineor could be relatively quite thick or could even be lensshaped or thelike within the scope of this invention.

In a desirable embodiment of this invention, a porous, and preferablymicroporous, filler or matrix-like member 22 of light-transmittingmaterial can be disposed within the central cavity of the cell 10. Thisporous filler or matrix can preferably be optically polished along itsouter surfaces 22.1 and 22.2 and the like and can be filled with aselected photochromic solution previously described. Most desirably, asillustrated in FIG. 2, the pores 24 of the filler do not extend betweenthe surfaces 22.1 and 22.2 in a direct perpendicular line but extendalong oblique or tortuous paths to assure that some of the solution 16can intercept substantially all light rays which may be incident uponthe filler surfaces 22.1 and 22.2, in a direction normal to suchsurfaces. This filler can serve to restrict any tendency of thephotochromic solution 1 6 to separate or form any precipitates and alsoserves to limit conventional diffusion of the solution with the cell.This particularly prevents any tendency for temporary streaking of thephotochromic solution during transition from one degree oftransmissivity to another and assures that the filtering means isuniformly transmissive throughout all of its parts at any time.

The filler 22 can be formed of any suitable porous or microporoustransparent material but is preferably selected to be of a materialhaving a refractive index corresponding quite closely to that of thephotochromic solution 16 employed therewith. Desirably, the filler cancomprise a very high silicate glass such as that sold by Corning GlassWorks of Corning, New York under the trade name porous Vycor.

It should be understood that particular embodiments of the variabledensity light filtering means provided by this invention have beendescribed by way of illustration but that this invention includes allmodifications and equivalents thereof which fall within the scope of theappended claims.

What I claim is:

1. A variable density light-filtering means comprising a solution of thereversible reducing agent stannous chloride, said solution having a pHadjusted to prevent precipitation of stannous and stannic ions, and athiazine dye adapted to be colorless in said solution in the absence ofultraviolet radiation and to be colored in said solution underultraviolet irradiation.

2. A variable density light-filtering means comprising a solution of thereversible reducing agent stannous chloride, said solution having a pHadjusted to prevent precipitation of stannous and stannic ions, and athiazine dye selected from the group consisting of Azure A, Azure B,Azure C, Toluidine Blue 0, Methylene Blue, New Methylene Blue andThionine, said solution being colorless in the absence of ultravioletradiation and being colored under ultraviolet irradiation.

3. A variable density light-filtering means comprising a solution of thereversible reducing agent stannous chloride, said solution having a pHadjusted to prevent precipitation of stannous and stannic ions, and athiazine dye selected from the group consisting of Azure A, Azure B,Azure C, Toluidine Blue 0, Methylene Blue, New Methylene Blue andThionine sealed within a transparnt cell in the absence of extraneousoxidizing agents, said solution being colorless in the absence ofultraviolet radiation and being colored under ultraviolet irradiation.

4. A variable density light-filtering means comprising a solution of thereversible reducing agent stannous chloride, wherein said solution is anacidified aqueous solution having a pH less than approximately 1.3 and athiazine dye selected from the group consisting of Azure A, Azure B,Azure C, Toluidine Blue 0, Methylene Blue, New Methylene Blue andThionine sealed within a transparent cell in the absence of extraneousoxidizing agents, said solution being colorless in the absence ofultraviolet radiation and being colored under ultraviolet irradiation.

5. A variable density light-filtered means comprising a solution of thereversible agent stannous chloride, wherein said solution is an alcoholsolution having a pH less than approximately 7 and a thiazine dyeselected from the group consisting of Azure A, Azure B, Azure C,Toluidine Blue 0, Methylene Blue, New Methylene Blue and Thionine sealedwithin a transparent cell in the absence of extraneous oxidizing agents,said solution being colorless in the absence of ultraviolet radiationand being colored under ultraviolet irradiation.

6. A variable density light-filtering means comprising a solution of thereversible reducing agent stannous chloride, said solution having a pHadjusted to prevent precipitation of stannous and stannic ions and athiazine dye selected from the group consisting of Azure A, Azure B,Azure C. Toluidine Blue 0, Methylene Blue, New Methylene Blue andThionine sealed within a transparent cell in the absence of extraneousoxidizing agents, wherein the concentration of said dye is greater thanabout 2.81 10 M and the concentration of said stannous chloride is atleast about 5 times greater than the concentration of said dye, saidsolution being colorless in the absence of ultraviolet radiation andbeing colored under ultraviolet irradiation.

7. A variable density light filter comprising a solution of a thiazinedye selected from the group consisting of Azure A, Azure B, Azure C,Toluidine Blue 0, Methylene Blue, New Methylene Blue and Thionine in aconcentration greater than about 2.81 X 10 M and the reversible reducingagent stannous chloride in a concentration at least about 5 timesgreater than the concentration of said thiazine dye, said solutionhaving a pH adjusted to prevent precipitation of stannous and stannicions, said solution being disposed within the pores of a poroustransparent member, said transparent member being sealed within atransparent cell in the absence of extraneous oxidizing agent, saidsolution being colorless in the absence of ultraviolet radiation andbeing colored under ultraviolet irradiation.

8. A variable density light filter comprising a solution of a thiazinedye selected from the group consisting of Azure A, Azure B, Azure C.Toluidine Blue 0, Methylene Blue, New Methylene Blue and Thionine in aconcentration greater than about 2.81 10- M and the reversible reducingagent stannous chloride in a concentration at least about 5 timesgreater than the concentration of said thiazine dye, said solutionhaving a pH adjusted to prevent precipitation of stannous and stannicions, said solution being disposed within the pores of a microporoussilicate glass matrix member having opposite surfaces thereof opticallyfinished, said matrix member being sealed within a transparent cell inthe absence of extraneous oxidizing agents, said solution beingcolorless in the absence of ultraviolet radiation and being coloredunder ultraviolet irradiation.

References Cited UNITED STATES PATENTS 3,266,370 8/1966 Marks et a1.350- 2,324,060 7/1943 Boughton 96-60 X 2,335,659 11/1943 Fraenckel etal. 178-7.5 3,121,632 2/1964 Sprague et a1. 96-48 OTHER REFERENCESEllis, C. and Wells, A. A., The Chemical Action of U1- traviolet Rays,N.Y. Reinhold, 1941, p. 631, QD 601, E45.

Menzies, D. W., Photo-Sensitivity of Thiazine Dyes. Nature, vol. 191,No. 4787, pp. 505-506, July 29, 1961, Q1, N2.

DAVID SCHONBERG, Primary Examiner.

T. H. KUSMER, Assistant Examiner.

1. A VARIABLE DENSITY LIGHT-FILTERING MEANS COMPRISING A SOLUTION OF THEREVERSIBLE REDUCING AGENT STANNOUS CHLORIDE, SAID SOLUTION HAVING A PHADJUSTED TO PREVENT PRECIPITATION OF STANNOUS AND STANNIC IONS, AND ATHIAZINE DYE ADAPTED TO BE COLORLESS IN SAID SOLUTION IN THE ABSENCE OFULTRAVIOLET RADIATION AND TO BE COLORED IN SAID SOLUTION UNDERULTRAVIOLET IRRADIATION.